1. Field of the Invention
This invention relates to the field of DC-DC converters, and particularly to methods of mitigating the adverse effects of electromagnetic noise sources on such converters.
2. Description of the Related Art
An important class of DC-DC converter uses the instantaneous output voltage to establish the duty ratio required for maintaining a regulated output. This class includes: 1) converters which do not employ a voltage-error amplifier, e.g., the various valley-voltage and peak-voltage regulators, and the hysteretic regulator; and 2) converters that employ the “Vsquare” technique (described, for example, in U.S. Pat. No. 5,770,940).
An example of such a converter is shown in FIG. 1. A switching element 10 is connected between an input voltage Vin and a node 12, and an inductor L is connected between node 12 and an output terminal 14 at which a regulated output voltage Vout is provided. Switching element 10 is turned on and off periodically. When switching element 10 is closed, (Vin−Vout) is connected across inductor L, such that the current in L (IL) increases. When switching element 10 is open, IL continues flowing in rectifier diode D, with IL decreasing due to the voltage (−Vout) applied across L. This results in IL having a sequence of rising and falling segments; i.e., essentially a dc current with a superimposed triangular ripple current. Inductor current IL feeds a load network comprising a capacitance Cout and a load 16. Cout is selected to have an impedance at the switching frequency that is much less than that of load 16. As such, the ripple component of IL flows mostly in Cout, and the dc component flows in the inductor; however, a small ripple voltage component will be present in Vout due to IL'S ripple current component.
The switching of element 10 is controlled by a switching control circuit 18, which cycles element 10 on and off once per switching cycle. The common characteristic of converters which use the instantaneous output voltage to establish duty ratio is that a feedback voltage Vfb representative of Vout—typically produced at a feedback node 20 using a resistive divider (R1 and R2) connected between output terminal 14 and the converter's local ground—is connected directly to one of the inputs of a comparator A1. When Vfb drops below and/or rises above a reference voltage Vref applied at the comparator's other input, the comparator changes state. Depending on the particular regulation technique employed, this change in state is used to affect the “on” time interval, the “off” time interval, or both the “on” and “off” time intervals of the switching cycle. For example, the regulator in FIG. 1 employs constant-on-time valley-voltage control. A monostable multivibrator (MMV) 22 is triggered when Vfb falls below Vref, which toggles the MMV's Q output and closes switching element 10 for a fixed (constant) time interval.
These converters are often operated in the immediate vicinity of electromagnetic interference (EMI) noise sources, such as other DC-DC converters. Another switching converter can emit a quickly-varying magnetic field, which can be coupled (via mutual inductance) into feedback voltage Vfb as noise. In addition, noise of electrical origin—such as the quickly-changing voltage at the switched node of a converter—can be coupled to feedback node 20 via stray capacitance. Noise in the feedback voltage can lead to jitter, undesirable frequency synchronizations, premature switching, or other malfunctions.
One prior art solution to this problem (shown in FIG. 1) is to add a filter capacitor Cf between feedback node 20 and ground. The capacitor together with divider resistors R1 and R2 form a low-pass RC filter, which attenuates the high-frequency components of noise picked up by the feedback divider. Furthermore, the filter capacitor together with stray capacitance form a capacitive divider, which attenuates the noise on the feedback node.
This solution suffers from several deficiencies. Due to the integrating effect of the RC filter, the magnitude of the ripple voltage component will be reduced. The reduced ripple might be insufficient to ensure jitter-free switching, especially if the unfiltered value of the ripple was already small. Secondly, the RC filter introduces a phase shift into the feedback voltage. This phase shift might reduce the stability margin of the converter, and the converter might become unstable as a result.